Adb --help

-d - directs command to the only connected USB device
returns an error if more than one USB device is present.
-e - directs command to the only running emulator.
returns an error if more than one emulator is running.
-s <specific device> - directs command to the device or emulator with the given
serial number or qualifier. Overrides ANDROID_SERIAL
environment variable.
-p <product name or path> - simple product name like 'sooner', or
a relative/absolute path to a product
out directory like 'out/target/product/sooner'.
If -p is not specified, the ANDROID_PRODUCT_OUT
environment variable is used, which must
be an absolute path.
devices [-l] - list all connected devices
('-l' will also list device qualifiers)
connect <host>[:<port>] - connect to a device via TCP/IP
Port 5555 is used by default if no port number is specified.
disconnect [<host>[:<port>]] - disconnect from a TCP/IP device.
Port 5555 is used by default if no port number is specified.
Using this command with no additional arguments
will disconnect from all connected TCP/IP devices.

adb backup [-f <file>] [-apk|-noapk] [-shared|-noshared] [-all] [-system|-nosystem] [<packages...>]
- write an archive of the device's data to <file>.
If no -f option is supplied then the data is written
to "backup.ab" in the current directory.
(-apk|-noapk enable/disable backup of the .apks themselves
in the archive; the default is noapk.)
(-shared|-noshared enable/disable backup of the device's
shared storage / SD card contents; the default is noshared.)
(-all means to back up all installed applications)
(-system|-nosystem toggles whether -all automatically includes
system applications; the default is to include system apps)
(<packages...> is the list of applications to be backed up. If
the -all or -shared flags are passed, then the package
list is optional. Applications explicitly given on the
command line will be included even if -nosystem would
ordinarily cause them to be omitted.)

- If <directory> is not specified, both /system and /data partitions will be updated.

- If it is "system" or "data", only the corresponding partition
is updated.

environmental variables:

ADB_TRACE - Print debug information. A comma separated list of the following values
1 or all, adb, sockets, packets, rwx, usb, sync, sysdeps, transport, jdwp
ANDROID_SERIAL - The serial number to connect to. -s takes priority over this if given.
ANDROID_LOG_TAGS - When used with the logcat option, only these debug tags are printed.